1. Field of the Invention
The present invention relates generally to an electroconductive resin and particularly to a composition useful for forming an electroconductive resin, the composition comprising a resin and a vapor-growth carbon fiber compounded with the resin and capable of being easily formed into a thin film, an electroconductive resin made from the composition, and a method of producing the electroconductive resin.
2. Description of the Related Art
With the advances in electronics techniques, electroconductive materials for electrostatic elimination and electromagnetic shielding which are light in weight, have high strengths and high electro-conductivity, and have a thin-film shape, and also compositions useful for forming electroconductive coating materials, electroconductive adhesives, and the above-mentioned electroconductive materials have been in more demand. As materials having the aforementioned properties excluding the electroconductive property, high polymer type materials can be used. However, almost all the high polymer type materials have insulating properties. Thus, methods for rendering an electroconductive property to such materials have been investigated.
According to known methods of rendering an electroconductive property to high polymer type materials, generally, electro-conductivity-rendering substances such as carbon black and metallic type materials are dispersed and contained in the high polymer type materials. However, to obtain the required electroconductive property, it is necessary to add large amounts of conductivity-rendering materials. In the case in which metallic type materials are added, problems occur in that the weights of the formed compounds are very large in general, and the electroconductive properties tend to decrease because of time-dependent oxidation. Moreover, if a material is selected in which deterioration of the electroconductive property is suppressed, the cost becomes high. Thus, the selection of such a material is unsuitable for practical applications.
Referring to the addition of carbon black as a conductivity-rendering material, it is very difficult to uniformly disperse carbon black in a high polymer type material. For example, for electroconductive resin composite materials containing carbon particles such as carbon black or the like, there is a disadvantage in that the structure of the carbon black may be broken when the carbon black is kneaded with resins, or when the composite materials are molded into predetermined shapes, so that the electric resistances are easily varied. It is difficult to obtain desired electric resistances by use of carbon black (see Column of Prior Art and so forth of Japanese Examined Patent Application Publication No. 02-38614 (Patent Document 1)).
To solve the above-described problems, a method has been proposed in which a crushed vapor-growth type carbonaceous material is mixed with different types of synthetic resins, and then kneaded to attain dispersion (see Patent Document 1), and a method in which graphitized vapor-growth carbon fiber and carbon black are mixed with a synthetic resin, and kneaded by means of a mechanical kneading machine such as a two-roll mill, a kneader, an internal mixer, a Banbury mixer, or the like. Thus, a conductive resin composition is produced, and thereafter, is formed by pressing into a sheet (see Column “Means for Solving the Problems” and so forth of Japanese Unexamined Patent application Publication No. 07-997730 (Patent Document 2)).
However, the above-described methods of kneading to attain dispersion have the following problems. Since vapor-growth type carbonaceous materials have a large aspect ratio in general, the dispersion is extremely insufficient, and thus, a stable conductive property is obtained with great difficulty. Moreover, with respect to sheeting, after a conductive resin composition is produced, the composition is formed by pressing into a sheet or the like. Therefore, according to this method, it is difficult to form the composition into a very small, homogeneous sheet or thin film.